Typical dynamic speakers include a diaphragm and a voice coil motor which causes piston motion for reciprocating the diaphragm to produce sounds. The typical dynamic speakers function substantially as a point sound source and exhibit a wide directivity at low frequencies but exhibit a sharp directivity over a frequency range equal to or higher than a frequency at which the diameter of a bore of the diaphragm is substantially equal to a half-wavelength of the reproduced sounds. Thus, small speakers using a diaphragm having small bores are used to reproduce sounds at high frequencies.
This also applies to dynamic microphones whose operation principle is reverse to that of the dynamic speakers. That is, small microphones using a diaphragm having a small bore are used to pick up high frequencies with a wide directivity.
In riffell speakers, in contrast, a diaphragm is constituted by a pair of rectangular curved plates, and the directivity is wide at middle and high frequencies. Also, sounds produced by the riffell speaker are radiated in a widthwise direction along a direction of curve of the diaphragm and hardly radiated in a vertical direction. Thus, it is possible to consider that an ideal sound space can be provided by arranging the riffell speakers in a row in the vertical direction as line array speakers.
Patent Documents 1 and 2 disclose conventional riffell speakers.
Patent Document 1 discloses a speaker in which a conductor pattern as a voice coil is printed on a central portion of a polymeric resin film, and the central portion is folded and bonded to form a diaphragm which includes first and second curved vibration portions and a planar plate portion having the conductor pattern, the planar plate portion and first and second curved vibration portions being formed integrally with each other. The planar plate portion of the diaphragm is disposed in a magnetic gap formed in a magnetic circuit, and distal edges of the first and second curved vibration portions are secured to a supporter.
Patent Document 2 discloses a speaker in which a central portion of a diaphragm is folded so as to form a recessed portion in which a flat voice coil wound in an oval annular shape is disposed in two magnetic gaps that are spaced apart from each other in an up and down direction. Also in this speaker, an outer peripheral portion of the diaphragm is secured to an annular frame.